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U.S. ENVIRONMENTAL PROTECTION AGENCY
SURVEILLANCE AND ANALYSIS DIVISION
TECHNICAL SUPPORT BRANCH
REGION VIII

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I DESIGN CONSIDERATIONS
FOR
AEROBIC DIGESTERS
by
BOB A. HEGG
(1)
and
KERWIN L. RAKNESS
(2)
(1)	At the time this report was prepared, Mr. Bob A. Hegg
was employed as Chief of the Operation and Maintenance
Section, USEPA, Denver, Colorado. At present, he is
employed as Chief Sanitary Engineer, M & I Incorporated
Consulting Engineers, Fort Collins, Colorado.
(2)	At the time this report was prepared, Mr. Kerwin L.
Rakness was employed as Sanitary Engineer, Operation
and Maintenance Section, USEPA, Denver, Colorado.
At present, he is employed as Sanitary Engineer, FMC
Corp. , Environmental Equipment Division, Englewood,
Colorado.

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TABLE OF CONTENTS
Page
I. Introduction		1
II. Purpose and Scope		1
III.	Summary of Design Considerations		2
A.	General Information		2
1.	Process Description		2
2.	Process Operation		3
3.	Process Reactions		5
4.	Design Factors		8
5.	Advantages of Aerobic Digestion		9
6.	Disadvantages of Aerobic Digestion. ...	9
B.	Unit Sizes		10
1.	Design Factors		10
2.	Estimating Digester Loadings		17
3.	Example Calculations		23
C.	Oxygen Requirements		29
1.	Efficiency Considerations		29
2.	Waste Sludge Considerations		32
3.	Mode of Operation Considerations		33
4.	Summary of Oxygen Requirements		35
D.	Supernatant Removal		36
E.	Tank Construction Considerations		40
F.	Operational Considerations		41
G.	Ultimate Disposal Considerations		43
IV.	Appendices		A-l
A.	References		A-2
B.	Additional References		A-4

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LIST OF FIGURES AND TABLES
Page
Figure 1 Digester Flow Schematic		12
Table 1 Hydraulic Detention Times		18
Table 2 Sludge Yield for Various Activated
Sludge Processes		20
Table 3 Clarifier Underflow Concentrations for
Various Activated Sludge Processes		24
Table 4 Values for Oxygen Transfer pt Selected
Air Supplies		31
Table 5 Summary of Oxygen Requirements		37
Table 6 Characteristics of Digester Supernptants ....	39
Table 7 Summpry of Ultimate Disposal
Considerations	 45
ii

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I.	INTRODUCTION
In recent years aerobic digestion of sludge has become an
increasingly popular method of stabilizing sludges prior to ultimate
disposal. Many of the design requirements, however, have been
based on laboratory studies and many of the parameters that affect
operation and performance of aerobic digesters have not been
studied in detail. As a result, specific design requirements that
would insure consistently good performance from all aerobic
digesters have not been developed.
Formerly, the Technical Investigation Branch of the Surveillance
and Analysis Division, and presently, the Operation and Maintenance
Section of the Air and Water Division, provides as one of its functions
technical assistance concerning the operation and maintenance of
wastewater treatment facilities. This assistance normally is in the
form of on-site operator training and plant evaluation. Another specific
objective of this program is to provide "feedback" concerning various
design and training deficiencies based on actual operational experiences
and observations. The following data on aerobic digesters represents
the result of such experience plus a summarization of other pertinent
design data.
II.	PURPOSE AND SCOPE
The purpose of this report is to summarize information concerning
the design of an aerobic digester.
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The parameters presented are based on information from various
literature sources and from actual plant operating experiences.
III. SUMMARY OF DESIGN CONSIDERATIONS
Aerobic digestion of sludges has become an increasingly popular
method of sludge handling, especially in smaller plants. This method
of sludge treatment competes favorably with the anaerobic digestion
process because aerobic digestion does not require extensive process
control or equipment (i. e. controlled temperatures, pH. alkalinity,
etc.). This does not mean that aerobic digesters will perform
satisfactorily without some process control or without proper design.
The emphasis in the following summary will be to provide design
considerations to insure satisfactory results using aerobic digestion.
A. GENERAL INFORMATION
1. Process Description
Aerobic sludge digestion is a process in which waste sludges
are subjected to aeration by various means to oxidize the
organic matter thus reducing the amount of sludge and making
it less objectionable aesthetically. The process uses primarily
the endogenous respiration (auto-digestion) phase of metabolism
to convert cell protoplasm and other biologically degradable
matter to carbon dioxide, water, and ammonia. Ammonia is
further converted sequentially to nitrite (NO-) snd nitrate (NO-).
2	3
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After a period of time (days) a final material is produced
that consists of inorganic solids and organic solids that
resist further biological destruction. This material or
sludge, is suitable for ultimate disposal (land application,
land fill, incineration, etc.). All types of sludges have
been subjected to aerobic digestion including primary sludge,
waste activated sludge, trickling filter humus, combinations
of primary and secondary sludges and industrial sludges.
2. Process Operation
Sludge is normally wasted to an aerobic digester from either
the primary or secondary clarifier or both. In some instances
a sludge thickener is used to concentrate the sludge prior to
discharge to the digester. In most secondary plants, secondary
sludge is wasted to the primary clarifiers and the combined
secondary sludge and primary sludge is then wasted to the
aerobic digester.
Sludge can be wasted to an aerobic digester continuously or
intermittently (batch operation). If sludge is wasted to an
aerobic digester continuously, normally constant supernatant
removal is required. To obtain constant supernatant removal
a quiescent zone in the digester must be provided to allow for
solids liquid separation.
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In practice effective, continuous removal of supernatant is
difficult to achieve due to the turbulence caused by aeration
in the aerobic digester which can effect the quiescent zone.
Solids liquid separation can become inefficient in these cases
and as a result many solids are contained in the supernatant.
To avoid the adverse effects that can be associated with con-
tinuous supernatant removal, many aerobic digesters are
operated on the batch process approach. The air to the
digester is shut off to allow time for solids liquid separation.
Supernatant is then decanted from the digester and the air is
turned back on. If diffused air or floating mechanical aerators
are used, the liquid level in the aerobic digester is not
critical and the digester may be refilled with waste sludge in
increments until it reaches a level where supernatant drawoff
is again required. If fixed mechanical aeration is used, the
digester must be refilled to an established level so that aeration
is effective.
Operational problems are encountered when supplying oxygen
to aerobic digesters. In diffused air systems clogging is
frequently encountered in batch operated aerobic digesters
because of the on-off operation associated with supernatant
removal. Clogging is also encountered in continuously operated
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digesters because of the high concentration of sludge solids.
In both instances, routine maintenance is required to insure
a continuous adequate supply of oxygen from a diffused air
system. Mechanical aeration systems become less effective
when rags and other debris collect on the blades of the aerators.
This also represents a maintenance problem and requires
routine attention in order to insure that adequate oxygen is
supplied to the digester.
Usually digested sludge is withdrawn from an aerobic digester
when a clear supernatant can no longer be decanted following ?
period of settling. Other criteria which indicate that the sludge
has been stabilized can be determined by laboratory testing.
These criteria are discussed later in the report.
3. Process Reactions
The reactions that take place in an aerobic digester depend in
part on the type of sludges wasted to the system. When primary
sludges are added to an aerobic digester, non cellular organic
solids are converted to activated sludge prior to undergoing
auto digestion. In other words, solids (cells) growth will occur
before endogenous respiration or cell destruction can proceed.
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Sludges wasted from activated sludge processes have already
been converted to cellular material and do not require a
preliminary growth phase before endogenous respiration or
cell destruction can proceed. This important concept must
be considered when designing an aerobic digester.
The following discussion includes the effect of primary sludges,
on the reactions that occur in the aerobic digestion process.
Aerobic bacteria require oxygen and organic matter in order
to live and reproduce. A simplified equation of this growth
process is:
O2 + microorganisms + organic matter =
activated sludge + CO2 + H2O
As aerobic digestion proceeds, the available food is depleted
and becomes inadequate for net activated sludge production.
At this point the aerobic bacteria begin to feed on cell proto-
plasm. The rate of cell destruction exceeds the rate of cell
synthesis and endogenous respiration (auto digestion) becomes
the predominant metabolic reaction. The organic matter
derived from cell protoplasm is converted to CO2, H2O, and
NHj. The ammonium ion is further converted to nitrites
(NO2) and nitrates (NO3).
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The two end products removed from the aerobic digester are
the supernatant liquor and the digested sludge solids. The
supernatant liquor from the aerobic digestion process is quite
different than the supernatant liquor produced from the aerobic
digestion process. A well digested sludge supernatant from an
aerobic digester will have a low suspended solids content and
will be quite clear. The 5-day biochemical oxygen demand
(BOD5) of an aerobic digester's supernatant is normally low
(less than 100 mg/l) since most of the BOD5 is used as food
by the "starving" aerobic bacteria. This low BOD5 content
reduces the impact of the supernatant liquor that may be
returned to the treatment system. The nutrient content is
generally quite high with the nutrients being in the form of
nitrates and phosphates.
The digested sludge drawn from an aerobic digester should
be dark brown, have a musty odor, and have good settling
characteristics. The solids content of the digested sludge
varies with the type of sludge fed to the digester. Ranges
have been reported (1) from 1. 5 percent to 6. 0 percent solids
by weight. The drainability (ability to dewater on drying beds)
of well digested aerobic sludges is generally better than that
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of anaerobically digested sludges. However, when sludge
is aerobically digested for short periods of time (less than
ten days) it has poor drainability characteristics.
Changes in various parameters can be used to determine the
degree of sludge digestion in aerobic digesters. Conversion
of ammonium ion to nitrates will occur, therefore a decrease
in NH| concentration and an increase in the nitrate concen-
tration should occur. Biological activity will decrease the
alkalinity of the digester contents. Also the pH of the contents
will decrease. The volatile solids concentration of the sludge
will decrease due to the endogenous respiration (auto digestion)
that takes place. All of these reactions will occur if digestion
is proceeding satisfactorily.
4. Design Factors
Many factors affect the design of an aerobic digester. Some
of the more important factors are: The type of sludge(s)
wasted to the digester; the size of the digestion facility; the
temperature of the digester contents; and the quantity of
oxygen required. These factors are discussed in detail later.
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Other factors affecting design are sludge loading rates,
ultimate disposal considerations, water quality criteria
(i.e. required nutrient removal), and various operational
considerations.
5.	Advantages of Aerobic Digestion
a.	Capital costs are normally lower than those of an
anaerobic system or other methods of handling sludge.
b.	The end product has no objectionable odors and is
biologically stable.
c.	Supernatant liquors have a lower BODg than those from
an anaerobic system, thus recycling of supernatant
liquors does not have as great an impact on the per-
formance of the treatment facility.
d.	Extensive process controls are not required and there-
fore operational problems are generally reduced.
6.	Disadvantages of Aerobic Digestion
a.	High power costs are required to supply the dissolved
oxygen.
b.	Methane gas is not produced and thus a useable end
product is not available.
c.	Temperatures greatly affect the performance by causing
increases or decreases in biological activity.
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d. Little information has been available on such important
design parameters as loading rates, air requirements,
ultimate disposal methods, effects of varying sludge
characteristics, sludge age requirements, and others.
This limits the use of aerobic digestion in many facilities
because engineers would not design a system based on the
limited information.
B. UNIT SIZES
The following section describes the rational used in selecting the
criteria for sizing an aerobic digester. A summary of this criteria
is presented in the various tables.
1. Design Factors
Several parameters have been used as a basis for designing
the size of an aerobic digester. Two of these parameters are
loading rate (lbs. VSS/day/cu. ft.) and solids residence time
(days). Of these parameters sufficient solids residence time
(sludge age) is generally accepted as the major criteria for
obtaining satisfactory sludge digestion. Although high loading
rates appear to affect the performance of the aerobic digester,
limited information is available on this parameter. Loading
rates have been studied (5) in the range of 0. 05 to 0. 25 lbs. VSS/
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day/cu. ft. When designing an aerobic digester, the loading
rate should be determined but normally an overloading problem
will not occur unless thickened sludge is fed to the aerobic
digester.
Solids residence time (sludge age) is the major criterion
selected for sizing an aerobic digester. Sludge age is defined
in many different ways making it difficult to compare various
values presented by different authors. The method used to
calculate sludge ages for this paper is outlined below. The
difference between sludge age and hydraulic detention time is
also discussed.
Figure 1 shows a typical flow schematic for an aerobic digester
and the symbols used to describe each parameter. The sludge
age is defined as the ratio of the weight of solids in the digester
to the weight of solids leaving the digester daily. Therefore,
during "steady state" conditions the equation for sludge age
using the symbols shown in Figure 1 is:
Sludge Age (days) =	ADV x ADC
(SPF x SPC) + (USF x USC)
It is noted that if the digester is working properly, a minimum
amount of solids should leave the digester in the supernatant
liquor. Therefore, the term (SPF x SPC) should approach zero
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FIGURE 1
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
August 1974
DIGESTER FLOW SCHEMATIC
SINGLE STAGE
AEROBIC DIGESTER


SPF @

ADV
SPC
1 XSF @
@

W XSC

1
ADC
USF @

USC
WHERE:
XSF	=	waste sludge flow (gpm or gpd)
XSC	=	waste sludge concentration (mg/l)
ADV	=	aerobic digester volume (gal)
ADC	=	mean aerobic digester solids concentration (mg/l)
SPF	=	supernatant flow (gpm or gpd)
SPC	=	supernatant solids concentration (mg/l)
USF	=	sludge flow to ultimate disposal (gpm or gpd)
USC	=	sludge solids concentration (mg/l)

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and the equation for sludge age simplifies to:
Sludge Age = ADV x ADC
USFxUSC
The sludge age or solids residence time will always be equal
to or greater than the hydraulic detention time due to the
effect of solids destruction by digestion and solids accumu-
lation by drawing off supernatant. Hydraulic detention time
can be calculated as:
Hydraulic Detention Time (days) = ADV or ADV
SPF + USF XSF
Since:
XSF^SFL + USF
The XSF will normally be greater than the sum of SPF + USF
due to evaporation.
Since sludge age is an important parameter in determining
whether an aerobic digester performs satisfactorily or not,
and since it is difficult to accurately predict the parameters
required to calculate sludge age, the most common parameter
used in the design of an aerobic digester is hydraulic detention
time. Therefore, if a sludge age is selected and a digester is
sized using the hydraulic detention time equal to the selected
sludge age. the design should be adequate since the sludge age
will always be greater than or equal to hydraulic detention time
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It is suggested that an aerobic digester be sized using the
hydraulic detention time equal to a selected sludge age.
Many factors affect the selection of the sludge age to be used
in designing an aerobic digester, and as a result, numerous
values have been reported. Some authors have used total
sludge age (i. e., sludge age in the secondary process plus the
sludge age in the aerobic digester). Others (11, 15)consider
only the sludge age in the aerobic digester.
In order to select the appropriate sludge age, two important
factors which significantly affect sludge digestion should be
considered. These factors are temperature and the type of
process preceding the digester.
Low temperature reduces biological activity and increases
the length of time required to stabilize sludge aerobically.
Since these cold temperature portions of the year are critical,
it is important to incorporate these conditions in the design of
the digester. Lawton (5) noted that temperature has an appre-
ciable effect with short detention times, but this effect decreased
considerably as the detention time lengthened. Experience at
the Trinidad. Colorado, Wastewater Treatment Pl^nt (6) showed
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that a 5° C decrease in sludge temperature, (from 15°C to
10°C) greatly inhibited biological activity as evidence by an
increase in the dissolved oxygen concentration from almost
zero to 2-3 mg/l, and by the fact that solids liquid separation
could no longer be accomplished. Thus when low temperatures
are encountered either increased sludge age accomplished by
increasing the volume of the digester or additional equipment
to maintain the temperature at 15°C or greater should be pro-
vided.
The second factor which significantly affects sludge digestion
is the type of process preceding the digester (i. e., the type
of sludge wasted to the digester). For example, a digester that
receives both primary and secondary sludge must be larger and
provide a longer detention time (sludge age) in order to produce
a stabilized sludge. A longer detention time is required because
the primary solids must first be converted to "activated solids"
before auto-digestion can proceed. Even if primary sludge is
not wasted to an aerobic digester, care must be taken in selecting
the required detention time because various activated sludge
processes yield sludges that have experienced varying degrees
of endogenous respiration. For example, a high rate process
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(normally associated with solids having p low sludge age)
produces a sludge that has undergone a limited amount of
endogenous respiration; whereas, the extended aeration
processes (normally associated with solids having a high
sludge age) produces a sludge that has undergone a consid-
erable degree of endogenous respiration. To include these
varying degrees of endogenous respiration, some authors
suggest using the total sludge age as the design parameter
(i. e., sludge age in the secondary process plus the sludge
age in the aerobic digester). Ahlberg and Boyko (1) suggest
that a minimum total sludge age of 45 days may be acceptable
for the design of an aerobic digester.
The more conventional requirement for detention time in the
aerobic digester has been 15 days hydraulic detention time.
Others (2, 11) have suggested that 10 - 15 days hydraulic
detention time is satisfactory if thickened activated sludge is
wasted to the digester. Twenty days hydraulic detention time
has been recommended when the waste sludge contains primpry
sludge solids. Table 1 presents a summary of suggested aerobic
digester hydraulic detention times (i. e., minimum sludge ages)
for various types of processes and various combinations of
waste sludges.
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TABLE I
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
AUGUST. 1974
HYDRAULIC DETENTION TIMES
Treatment Process and
Type of Sludge
Suggested Detention Time
in Aerobic Digester
(Days)
Remarks
Waste Activated Sludges Alone
(Temperature effects not
included - see 1 below)

High Rate Processes
20-25
If a minimum total sludge age of
Conventional
15-25
45 days can be established with a
Step aeration
15-25
lesser hydraulic detention time in
Contact stabilization
15-20
the digester, then the lesser time
Extended aeration
15-20
in the digester may be used.
Primary + Waste Activated
20-30

Primary Sludge Alone
>20

Trickling Filter Sludge
20-30

Other Sludges (industrial, etc.)
—;	:	;	=	r:		r;	
It is recommended that pilot studies
be conducted on unique sludges to ensure
adequate digestion facilities.
provided. It is suggested that an additional 5 to 10 days hydraulic detention time be included in the
design when low temperature operations are expected. Also it is suggested that design steps be taken
to minimize the periods of low temperature operation.

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2. Estimating Digester Loadings
Once the hydraulic detention time has been selected, the next
step in determining the unit size of an aerobic digester is to
determine the volume of sludge that will be wasted to the
digester (hydraulic loading). Hydraulic loading is a function
of two factors: the quantity of solids produced by the treatment
process, and the concentration at which these solids are wasted
to the digester.
For a given wastewater stream, the quantity of solids produced
(sludge yield) by a treatment process depends on the type of
process used. Two of the most common types of treatment
systems will be analyzed for sludge yields. The first system
is the conventional type of plant that includes primary and
secondary treatment, and the second system will include such
processes as extended aeration where no primary treatment is
provided.
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The quantity of sludge wasted to the digester from a
conventional (primary plus secondary) plant is equivalent
to the total quantity of primary sludge plus the total quantity
of secondary sludge. The total quantity of primary sludge
is normally equal to 55 to 75 percent of the incoming suspended
solids. The quantity of sludge from the secondary process
depends on the strength of the incoming sewage and the type
of secondary treatment process. Table 2 shows yield figures
(lbs of suspended solids produced per lbs of BOD5 removed)
for various types of activated sludge processes. These
values can be used as a guide if pilot data is not available.
The second type of system associated with aerobic digestion
are the contact stabilization and extended aeration processes
that normally don't have primary treatment preceding the
secondary facilities. The sludge yield for these processes
are outlined in Table 2. However, if these processes do not
have separate primary treatment, additional sludge must be
handled as a result of biologically inert volatile and suspended
matter that enters the plant in the influent. One author (7)
suggests that normal domestic sewage contains about
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TABLE 2
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
AUGUST, 1974
SLUDGE YEILD FOR VARIOUS ACTIVATED SLUDGE PROCESSES (1)
Avg.MLSS Avg. Avg. Loading Predicted (2)	Suggested
Concen- Aeration Factor	Process Yield Design Yield
tration Time lb. BODg/ lbs. TSS Produced lbs. TSS Produced
High Rate
Processes
500
3
0.5-5.0
0.70-0.80
0.75
Conventional
2000
6-8
0.2-0.5
0.50-0.70
0.60
Step Aeration
2500
3-4
0.2-0.5
0.50-0.70
0.60
Contact (3)
Stabilization
Contact
3000
Reaeration
6000
0.25-0.5
2-6
0.2-0.5
0.50-0.70
0. 60
Extended
Aeration
4000
24
0.05-0.2
0.15-0.50
0.35
(1)	Values were calculated using following assumptions:
Y = yield coef. = 0.65 lb. VSS produced per lb. of BOD removed,
endogenous coef. = 0.05 lb. VSS destroyed per lb. of VSS system.
VSS/TSS = 0.80
Waste is typically domestic with limited industrial.
(2)	Industrial wastes may differ drastically and pilot studies are recommended in
order to determine sludge yield.
(3)	If these processes are not accompanied by primary treatment additional sludge
must be handled as a result of the biologically inert volatile and suspended
matter that enters the plant in the influent.
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125 mg/l of inert suspended solids. Another (8) states that
20-40% of the volatile influent suspended solids plus 20-30%
of the total influent suspended solids are biologically inert.
Therefore, to determine the quantity of sludge produced by a
process that does not have separate primary treatment both the
quantity of biologically inert solids that enter the plant and
the quantity of solids produced by the process must be calculated.
The sum of these is equal to the total quantity of sludge produced
by the system.
The next important step in determining the hydraulic load to the
aerobic digester is determining the concentration at which the
solids are wasted to the digester. For primary sludges the under-
flow concentration usually varies from 3 percent (30, 000 mg/l)
to 10 percent (100, 000 mg/l) by weight. Normally a 5 percent
(50,000 mg/l) underflow concentration for a primary sludge is
assumed as a typical value. If industrial wastes, chemicals, or
waste secondary sludges are added to the primary clarifier,
then the resulting underflow concentration may either increase
or decrease. This change in underflow concentration must be
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considered in design. For example, if waste activated sludge
is added to the primary clarifier influent, the underflow con-
centrations of primary plus secondary sludges have been found
in the range of 1. 5 (15,000 mg/1) to 5 (50,000 mg/l) percent.
For design it is suggested that an underflow concentration of
2 (20, 000 mg/l) to 3 (30, 000 mg/l) percent be used.
Little information is published giving clarifier underflow con-
centrations that can be expected with the various modifications
of the activated sludge process. However, most sources give
typical values for return sludge flow rates, sludge volume index,
and mixed liquor suspended solids concentrations. Using the
values and the physical mixing formulas, underflow concentration
can be predicted. Table 3 summarizes the various underflow
concentrations associated with several modifications of the
activated sludge process. In addition, suggested design under-
flow concentrations are given. Processes whose parameters
differ from the mixed liquor concentrations, loading rates, or
detention times shown should be re-evaluated to determine the
expected underflow concentration. When the required hydraulic
detention time, quantity of solids produced, and underflow
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TABLE 3
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
AUGUST 1974
CLARIFIER UNDERFLOW CONCENTRATIONS FOR
VARIOUS ACTIVATED SLUDGE PROCESSES


Avg.
Avg. Loading


Predicted
Suggested
Treatment
Avg. ML
Aeration
Factor
Avg. Sludge
Avg. Return
Range of
Design
Process
Concentration
Time
lb. BOD5/
Vol. Index
Sludge Rate
Underflow Cone.
Underflow Cone
(Activated Sludge)
(mg/l)
(hrs. )
lb. MLVSS/day
(ml/gm)
% of Flow
(mg/l)
(mg/l)




156-1000(1)
10-50(3)


High Rate
500
3
0.5-5.0
100- 300(2)
Avg. 20
1000-10. 000
3500




63-156
15-75


Conventional
2000
6
0. 2-0. 5
80-150
Avg. 30
4700-15. 800
7000




63-156
20-75


Step Aeration
2500
3-4
0. 2-0.5
80-150
Avg. 50
5800-15.000
7000

Contact Zone






Contact
3000 0.25-0.5

63-156
50-150


Stabilization
Reaeration Zone

0. 2-0.5
80-150
Avg. 100
5000-12. 500
6000

6000
2-6





Extended



54-170
50-200


Aeration
4000
24
0. 05-0. 2
Avg. 100
Avg. 100
5900-25. 000
9000
(1)
(2)
(3)
Top valves taken from reference (8)
Bottom valves taken from reference (9)
Valves taken from reference (10)

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concentration are determined, the size of the aerobic digester
can be calculated. An example calculation is presented in the
next section.
It is noted that the data presented in this section of the report
assumed that no thickening device was being used. If sludge
thickeners (i. e.. air flotation units, centrifuges, gravity
thickeners, etc.) are used, an increase in solids concentrations
wasted to the digester can be expected and the digester capacity
can be reduced accordingly.
3. Example Calculations
The following calculations are presented to demonstrate a
method of determining the required size of an aerobic digester
as suggested in this report.
Problem:
Design the unit size of an aerobic digester for a one million
gallon per day treatment plant that has primary treatment
followed by a conventional activated sludge system.
Given Parameters:
Influent flow = 1 mgd (0.044 m^/sec)
Influent BOD5 = 200 mg/l
Influent TSS = 250 mg/l
Allowable Effluent BOD5 and TSS = 20 mg/l
Sludge Temperature in Digester = 15° C
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Design - Alternative #1:
Assume separate waste sludge streams from the primary
and secondary clarifiers.
a)	Determine the volume of sludge removed from the
primary clarifier each day.
Assume 65% removal of influent TSS
Assume 5% (50, 000 mg/l) underflow concentration.
Therefore, the quantity of TSS removed in the primary
clarifier each day equals:
(1 MGD) (250 mg/l) (8. 34 lb/gal) (0. 65) = 1355 lb TSS/day
(614. 6Kg)
The volume of sludge wasted to the digester per day to remove
1355 lb. TSS at 5% underflow concentration equals:
(1352 lb TSS/day) (1/50, 000 mg/l) (1/8 .34 lb/gal) = 0. 0032
MGD = 3250 gal/day (1. 43 x 10"^ m^/sec)
b)	Determine the volume of excess activated sludge to be wasted
from the secondary clarifier each day.
Assume sludge yield of 0. 6 lbs TSS produced (Table 2) /o. 6 Kg)
lb BODg removed ( Kg /
Assume 7000 mg/l underflow concentration (Tpble 3)
Assume 35% removal of BODg in primary clarifier
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The quality of BOD,. removed in the secondary process
equals (the quantity of BOD^ in the influent) - (the quantity
of BOD5 removed in the primary + the quantity of BOD5
in the effluent) or:
(1 MGD) (200 mg/l) (8. 34 lb/gal) - (1 MGD) (200 mg/l)
(8. 34 lb/gal) (0. 35) + (1 MGD) (20 mg/l) (8. 34 lb/gal) =
(1668 lb/day) - (584 lb/day + 167 lb/day) =917 lb/day
(415.9 Kg)
The quantity of sludge produced in the secondary process
equals:
(917 lb BOD5 removed/day) (0. 6 lb TSS produced per
lb BOD5 removed) = 550 lb TSS produced/day (249.5 Kg)
The volume of sludge wasted to the digester per day to
remove 550 lb TSS at 7000 mg/l underflow concentration
equals:
(550 lb TSS/day) (1/7000 mg/l) (1/8. 34 lb/gal) =
. 009420 MGD = 9420 gal/day (4.13 x 10~^m^/sec)
c) Determine the volume of sludge wasted to the digester
each day.
The total volume of sludge wasted to the aerobic digester is
the sum of the volume wasted from the primary clarifier and
the volume wasted from the secondary clarifier or:
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= (3250 gal primary/day) + (9420 gal secondsry/day)
= 12, 670 gal/day (5. 56 x 10"^m^/sec)
d) Determine the required size of the aerobic digester.
Assume Hydraulic Detention Time equals 25 days,
(Table 1). The required volume of the digester
equals: (12670 gal/day)(25 day) = 317,000 gal (1200 m^)
Design - Alternative #2
Assume that the secondary sludge is wasted to the primary
clarifier and that the combined primary and secondary
sludge is then wasted to the digester.
a)	Determine the total pounds of sludge wasted to the
digester each day.
The total pounds of sludge wasted to the digester each day
is equal to the quantity of sludge removed by the primary
clarifier (see Alternative #1) plus the quantity of sludge
produced by the secondary process (see Alternative #1), or:
(1355 lbs TSS/day) + (917 lbs TSS/day) =
2272 lbs TSS/day (1030 Kg)
b)	Determine the total volume of sludge wasted to the
digester each day.
Assume 2. 5% (25, 000 mg/1) underflow concentration
for the combined sludges.
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The total volume of sludge wasted to the digester to remove
2272 lbs. TSS/day at 2. 5% underflow concentration equals:
(2272 lbs. TSS/day)(l/25,000 mg/l)(l/ 8. 34 lb/gal) =
. 0109 MGD = 10, 900 gal/day (4. 78 x 10~^m3/sec)
c) Determine the required size of the aerobic digester.
Assume Hydraulic Detention Time equals 25 days,
(Table 1). The required volume of the digester equals:
(10,900 gal/day)(25 days) = 273,000 gal (1034 m3)
Summary:
Based on the above analysis, the minimum volume for the
aerobic digester is obtained by using the primary clarifier as
a "thickener" for the secondary sludge and the unit should be
designed using this method of operation. It is important that the
flexibility to waste separately from both primary and secondary
clarifiers directly to the aerobic digester be provided (alternative
#1). In addition, flow control and measurement should be provided
for the waste activated sludge from the secondary process.
An alternative that could alter the volume of the aerobic
digester would be to provide for a thickener to handle the secon-
dary sludge prior to discharge to the digester. The important
aspect of a thickener is the flexibility it provides the operator
in controlling his plant. It is noted that in showing the example
calculations an assumption was made that the sludge temperature
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in the digester would always be greater than 15° C. If this
plant were to be constructed in a colder climate and equipment
was not provided to maintain a temperature in the digester of
at least 15° C, then additional capacity would be required.
C. OXYGEN REQUIREMENTS
Numerous parameters influence the oxygen requirements for an
aerobic digester. Parameters such as process preceding the
digester, solids concentration in the digester, two stage versus
single stage digestion, sludge detention time in the digester, temp-
erature, altitude, etc., must all be considered when designing the
air supply requirement. In this ection of the report, these para-
meters will be discussed and values for oxygen requirements for
various treatment processes will be given.
1. Efficiency Considerations
An aeration system oxygen transfer efficiency (Te = ratio of
oxygen transferred to the liquid to the oxygen supplied to the
system) greatly influences the adequacy or inadequacy of the
supply system. Table 4 shows the actual quantity of oxygen that
is supplied to the liquid at air (volume) flow rates of 20 cfm and
90 cfm per 1000 ft^ (9. 4 mVsec and 42. 5 m^/sec per 28. 3 m^)
of digester capacity and at various transfer efficiencies.
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TABLE 4
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
AUGUST 1974
VALUES FOR OXYGEN TRANSFER
AT SELECTED AIR SUPPLIES
Assumed
Transfer Eff. (1)
Te
%
Oxygen Transfer
at 20 cfm supply per
1000 ft of digester (2)
mg/l/hr
Oxygen Transfer
at 90 cfm supply per
1000 ft^ of digester (2)
mg/l/hr
5
16. 3
73.4
10
32. 6
146.9
15
49.0
220.4
20
65.3
293.8
25
81. 6
367. 2
(cfm x 0.472 = m^/sec) and (ft^ x 0. 0283 = m^)
(1)	Te = ratio of oxygen transferred to the liquid to the oxygen
supplied to the system.
(2)	Assumed:
Air Temperature = 32° F (0° C)
Ratio of oxygen to air -21%
Altitude = Sea Level
Not Considered:
Different altitudes, pressures, temperatures, "alpha" coefficients
and "beta" coefficients.
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It is important to note that the values presented in Table 4
do not include the expected decrease in oxygen transfer due
to the high solids concentration of the digester contents or
other characteristics of the wastewater. In addition, the
increase or decrease in oxygen transfer due to variations in
temperature and altitude are not considered.
The expected decrease in oxygen transfer due to the high
solids concentration of the liquid in the digester and other
characteristics of the wastewater is normally considered in
the form of "alpha" and "beta" coefficients. The "alpha"
coefficient is the ratio of the overall oxygen transfer coefficient,
K-^a, of the waste to that of water. The "beta" coefficient is
the ratio of the total solubility of oxygen in the waste to the
solubility of oxygen in water. Specific values for these
coefficients are not presented in this report.
It is suggested that the design of an aeration system specify
the quantity of oxygen to be supplied to the liquid in the digester
in terms of mg/l/hr. Once this quantity has been established,
the designer may select the size and type of aeration equipment.
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In most cases different equipment has different transfer
efficiencies, and if the quantity of oxygen as mentioned above
is selected, then it does not matter which piece of equipment
is furnished because that quantity of oxygen should be attained.
(Note: The design should insure that adequate mixing is also
provided.)
2. Waste Sludge Considerations
The type of sludge wasted to an aerobic digester also affects
the oxygen requirement. For example if waste activated
sludge only is wasted to an aerobic digester, the quantity of
air required to provide adequate oxygen and mixing is reported
to be 15 - 20 cfm per 1000 ft^ (7. 07 - 9. 43 m^/sec per 28. 3 m^)
of digester capacity (2. 11). However, if primary sludge is
wasted to the digester along with waste activated sludge, then
additional air is required to satisfy the oxygen demand due to
the increased activity in the digester. Loehr (12) reports that
the volume of air required increases almost ninefold. Burd (11)
suggests that a minimum supply of 90 cfm per 1000 ft^ (42. 5 m^/
sec per 28. 3 m^) of digester capacity be provided.
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The type of secondary process preceding the digester plso
influences the quantity of oxygen required because in some
cases the sludge wasted to the digester is more stabilized
and requires less oxygen (i. e. , sludge from an extended
aeration process) than in other cases (i. e. , sludge from a
high rate activated sludge process). Other factors may
explain the broad range of oxygen utilization rates reported
in the literature.
3. Mode of Operating Considerations
The mode of operation influences the quantity of oxygen
required because in most cases sludge that has just been
wasted to the digester has a high oxygen demand. Barnhart
(13) reports initial oxygen utilization rates of 60 mg/l/hr.
He also shows that the oxygen utilization rate decreases
rapidly during the first two days of digestion and decreases
much slower after the second day. Rates less than 20 mg/l/hr
after 10 days digestion were reported. Specific oxygen utili-
zation rates (i. e., the oxygen utilization rate divided by the
volatile or suspended solids concentration of the digester con-
tents) were reported by Barnhart to range from 10 mg/l/hr per
1000 mg/l of volatile suspended solids to 4 mg/l/hr per 1000
mg./l of volatile suspended solids after 10 days digestion.
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This apparent high initial oxygen demand becomes increas-
ingly important when designing a two-stage digestion system
or when designing a plug flow digestion system. In both cases
attempts should be made to satisfy the high initial oxygen
demand in the first stage of a two-stage digester or the head
end of a plug flow unit.
The solids concentration of the liquid in the digester also
influences the quantity of oxygen required in that higher solids
concentrations require more oxygen. Attempts have been made
to include the effect of the solids concentration on the quantity
of oxygen required by reporting specific oxygen utilization rates
(i. e., oxygen utilization rate divided by the suspended or volatile
solids concentration of the digester contents). Smith (14) reports
specific oxygen utilization r?tes of 3 mg/l/hr per 1000 mg/1 of
suspended solids. Benedict and Carlson (15) report specific
oxygen utilization rates during endogenous respiration of 5. 0
mg/l/hr per 1000 mg/1 of suspended solids at a temperature of
32° C, 3. 5 mg/l/hr per 1000 mg/1 of suspended solids ?t a
temperature of 17° C, *nd 1.0 mg/l/hr per 1000 mg/1 of sus-
pended solids at a temperature of 4° C. Unfortunately, only a
limited amount of information is available on the specific oxygen
utilization rate, but this does not decrease the importance of
solids concentration on the oxygen requirement.
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4. Summary of Oxygen Requirements
Many different parameters have been used to describe the
air or oxygen requirements for an aerobic digester. These
parameters have units varying from "ft^/min" of air to
mg/l/hr of oxygen per 1000 mg/l of suspended solids. Perhaps
the most common design criteria requires that the dissolved
oxygen in the aerobic digester be between 1-2 mg/l and that
adequate mixing be provided to keep the solids in suspension.
This criteria, like a volume requirement given in ft^/min per
1000 ft^ of digester capacity, does not necessarily insure an
adequate oxygen supply. Therefore, all units given in this
report will be described in units of mg/l/hr of oxygen (oxygen
utilization rates) that must be supplied to the liquid in the
digester.
Many different values for oxygen utilization rates have been
reported. Barnhart (13) indicated values from less than 20 to
60 mg/l/hr. Ahlberg and Boyko (1) show values from approxi-
mately 6 to 45 mg/l/hr. Standard design values indicate
requirements of 16 to 82 mg/l/hr (see Table 4). Values at the
Trinidad Wastewater Treatment Plant (6) ranged between 35
and 50 mg/l/hr.
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Suggested design values for oxygen requirements for various
types of treatment processes preceding the aerobic digester
pre presented in Table 5. The broad rsnges presented pre
necessary because of the Limited data ^vailpble. Specific
remarks concerning the use of the Tpble are also indicated.
Settling characteristics of digesting sludge deteriorate with
low residual dissolved oxygen levels and with increasing solids
concentrations. The effect low D. O. mpy have on settling
should be considered when sizing the aeration equipment. Good
settling characteristics are p must if the aerobic digester is to
be successful. This is especially true if the aerobic digester is
the only method of sludge disposal.
D. SUPERNATANT REMOVAL
Separation of the supernatant liquor and the digested sludge
solids is normally accomplished in smpller plants (less th^n
1 MGD or 0. 044 m^/sec) by using the entire digester or ? portion
of the digester as a sludge thickener. Larger plants may have
separate sludge thickening facilities in the form of gravity thick-
eners, centrifuges, etc.
An important criterion in good aerobic digester operation, is
obtaining a relatively clear supernatant that has a low suspended
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TABLE 5
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
August 1974
SUMMARY OF OXYGEN REQUIREMENTS
Treatment Process
and Type of Sludge
Suggested Range of Values
for Oxygen Supply(mg/l/hr)
Remarks
Waste Activated Sludges
(alone)

Digesters operating at high solids concentrations
should use upper portion of range (i. e. , solids
concentration greater thr>n 25, 000 mg/l).
High rate processes
Conventional
Step aeration
30 - 75
25 - 70
25 - 70
First stage digesters of a two-stage system
should provide a higher oxygen supply.
Mixing requirements must ?lso be met when
selecting aeration equipment.
Contact stabilization
Extended aeration
20 - 60
15 - 60
Complete mix digesters may consider the lower
values for oxygen supply.
Primary + Waste Activated
> 75
Primary sludge greatly increases the oxygen
demand and therefore requires a higher oxygen
supply (seq^Table 4 values for 90 cfm supply
per 1000 ft ).
Other Sludges (industrial)

It is recommended that pilot studies be conducted
on unique sludges to insure th?t an adequate
oxygen supply is provided.

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solids concentration. In most cases, the quality of the
digester supernatant can be directly related to the settle-
ability of the digesting sludge solids (i. e., poor settling
characteristics result in poor quality supernatants). The
main factors that affect the settleability of the sludge solids
are the dissolved oxygen concentration, the suspended solids
concentration, and the degree to which the sludge has been
stabilized. Low D. O. concentrations and high solids concen-
trations have an adverse effect on the settling characteristics
of the digesting sludge. Partially digested or unstabilized
sludges normally do not settle as well as waste activated
sludges that have not undergone any digestion, or as well as
thoroughly digested sludges. Since a high quality supernatant
is dependent upon these factors, it is important to design the
digester to achieve these conditions prior to considering the
design of supernatant removal facilities.
Supernatant removal facilities should have a flow measuring
device to measure the amount of supernatant withdrawn. A
flow measuring device is necessary in order for the operator
to conduct a flow and solids balance on his digester and thus
determine the solids residence time.
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Scum removal is recommended as part of the supernatant
removal facilities to retain the heavy brown scum and
grease that accumulates on the surface of the digester during
quiescent periods. Scum sprays are not recommended because
the water added to the digester displaces volume that could be
used for digesting sludge solids.
Supernatant drawoff facilities should be located as far as
possible from the point where undigested waste sludges are
added to the digester. This decreases the chances of short
circuiting, and decreases the turbulence near the drawoff
facility which can adversely affect supernatant quality. Many
types of drawoff facilities are available. The more common
facilities are multiple port drawoff tubes, hinged tubes that can
be raised or lowered manually, baffled weirs, etc.
Table 6 shows typical digester supernatant characteristics as
reported in various sources.
E. TANK CONSTRUCTION CONSIDERATIONS
Two important items that should be considered when designing
the location and configuration of an aerobic digester are temper-
ature and mixing.
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TABLE 6
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
August 1974
CHARACTERISTICS OF DIGESTER SUPERNATANTS
(1, 6)
Parameter
Reported Range
of Values
Typical
Average Value

pH (units)
5.5-7.7
<7. 0
BOD5 (mg/1)
9 - 1700
< 200
TSS (mg/1)
46 - 11, 500
<1000
Ammoni? Nitrogen (mg/1)
- - -
<10
Nitrate Nitrogen (mg/l)
- - -
>25
Total P (mg/l)
19 - 241
>10

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Heat losses in winter months normally have the most adverse
affect on aerobic digester performpnce. To minimize these
heat losses digesters could be constructed with a common wall
(i. e. , steel which allows heat transfer) with the activated
sludge aeration tanks. Earthen embankments or constructing
the tank below grade would aid in maintaining higher sludge
temperatures. Heat can also be added through the air supply
where a diffused air system is used for oxygen supply. Perhaps
the most important method of controlling heat loss is covering
the aerobic digester.
Mixing is an important factor that is influenced by the t?nk con-
figuration. For example, surface mechanical aerators are not
recommended for deep tanks unless bottom mixers are included.
Another tank construction consideration which would improve
operational flexibility is using multiple aerobic digestion tanks
when feasible. If two tanks are constructed, they should be inter-
connected to allow for either series or parallel operation.
F. OPERATIONAL CONSIDERATIONS
Several features should be included in the design of an aerobic
digester for operational control. These are flow measuring
devices and sampling points on the influent waste sludge flow
and the effluent waste sludge and supernatant liquor flows.
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These features would enable the operator to monitor the
quantity and quality of the flow streams entering or leaving
the digester. With this information, the operator would be
able to select and control an optimum mode of operation.
To insure adequate operation and control of the aerobic
digestion process, certain laboratory facilities and equipment
should be provided. One of the most important tests is to
determine the dissolved oxygen concentration of the digester
contents. The high solids concentration of the digester, however,
makes it difficult to routinely measure D. O. using the Winkler
method. Therefore, a portable dissolved oxygen meter and
probe is recommended to enable the operator to monitor the
D. O. concentration.
Another monitoring test is the determination of the pH of the
digester contents. The pH test can be used to indicate if digestion
is proceeding since during the process of aerobic digestion, the
pH will normally decrease from greater than 7. 0 to less than 6. 0.
Historically, a reduction in the VSS of the sludge has been used
to indicate the stability of the sludge and to monitor the perfor-
mance of the digester. Ahlberg and Boyko (1) have proposed that
rather than use the volatile suspended solids reduction, the
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specific oxygen uptake rate be used to determine the stability
of sludge. They have suggested that a well digested sludge
has a specific oxygen uptake rate in the range of 0. 5 to 1. 0
mg/l/hr of oxygen per 1000 mg/l of VSS.
Other tests and parameters which can be used to monitor and
control the operation of an aerobic digester are alkalinity,
nitrates, BOD5, TSS, and ammonia. In general, as digestion
proceeds, alkalinity, ammonia, and BOD5 will decrease and
nitrates will increase. Total suspended solids test results are
used to monitor increases or decreases in digester solids con-
centrations and to determine the concentration at which solids
are withdrawn for ultimate disposal.
G. ULTIMATE DISPOSAL CONSIDERATIONS
At small plants most aerobically digested sludges are disposed
in drying beds, in sludge lagoons, or hauled and spread directly
on the land. Each of these disposal methods are temperature
and weather dependent with problems being frequently encountered
during winter months. During the same winter period, aerobic
digesters are least efficient. This combination of decreased
efficiency during the winter, coupled with the problems of disposal
has caused many operational difficulties in small plants. When
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determining the method for ultimate disposal of the sludge,
it is important that critical periods, such «s winter operation,
be considered when sizing the disposal unit(s) or site(s).
Inadequate sludge handling facilities, even for short periods
of the year, can adversely affect the performance of the entire
wastewater treatment facility.
Larger plants have the same winter problems as the smaller
plants, but in many cases additional equipment is provided
such as sludge thickeners, centrifuges, filters, etc. , to help
alleviate some of the sludge handling and disposal problems.
When selecting and sizing an ultimate disposal unit or site
several factors must be considered. Two important factors
are quantity and volume of sludge to be disposed. Table 7
presents the range of values for suspended solids reduction
and concentration of gravity thickened sludge to aid in deter-
mining the quantity and volume of sludge that must be disposed.
Flow control devices, flow measuring devices, and sampling
locations should be provided on the ultimate disposal drawoff
lines from an aerobic digester enabling the operator to select-
ively control the quantity of sludge removed from the digester.
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TABLE 7
SUGGESTED DESIGN CONSIDERATIONS
FOR AEROBIC DIGESTERS
August 1974
SUMMARY OF ULTIMATE DISPOSAL CONSIDERATIONS
(1, 3, 4, 5, 6, 16)
Characteristics
Reported Range
of Values
Suggested Design
Values
(winter operation)

Total Suspended Solids
Reduction - %
5 - 45
5 - 15(1)
Solids Concentration
of Gravity-Thickened
Aerobic Sludge mg/1
20, 000 - 60, 000
25,000^
(1)	If adequate provisions pre m^de for controlling heat losses, higher
values may be considered.
(2)	If provisions are made for additional thickening equipment (centrifuges,
vacuum filters, air flotation, etc.) higher concentrations can be
achieved.
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IV APPENDICES
Appendix A References
Appendix B Additional References
A-l

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APPENDIX A
REFERENCES
1.	Ahlberg, N. R. and Boyko, B. I., "Evaluation and Design
of Aerobic Digesters," Journal-Water Pollution Control
Fed,, Vol. 44, No. 4, 634 (April 1972)
2.	Dreier, D. E. and Obma, C.A., "Aerobic Digestion of Solids, "
Walker Process Equipment Co. Bulletin No. 26-S-18194,
(January 1963).
3.	Ritter, Lewis E., "Design and Operating Experiences Using
Diffused Aeration for Sludge Digestion," Journal-Water Poll.
Control Fed., Vol. 42, No. 10, 1782 (October, 1970).
4.	Aerobic Digestion of Organic Waste Sludge, U. S. Environmental
Protection Agency, Water Pollution Control Research Series,
No. 17070 DAU, (December 1971).
5.	Lawton, G. W., Norman, J. D., "Aerobic Sludge Digestion
Studies, " Journal-Water Poll. Control Fed., Vol. 36, No. 4,
495 (April 1964).
6.	"Technical Assistance Project - Trinidad Wastewater Treatment
Facility - Trinidad, Colorado, " Report by Region VIII of the
U.S. Environmental Protection Agency, Denver, Colorado, No.
S & A/TSB-9.
7.	McKinney, R. E., Outline for Course Entitled - Biological
Treatment Technology, Sponsoroed by U. S. Environmental
Protection Agency, Water Programs Operations, National
Training Center, Cincinnati, Ohio, October 30 to November 10,
1972.
8.	Goodman, B. L. and Foster, J. W., "Notes on Activated Sludge, "
Smith and Loveless Corp., Division of Union Tank C?r Company,
Lenexa, Kansas, Second Edition (1969).
9.	Stewart, M. J., "Activated Sludge Process Variations - The
Complete Spectrum, " Water and Sewage Works, Vol. Ill, No. 4,
(April 1964).
A-2

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10.	Montgomery, J. A., "An Outline - Activated Sludge Waste
Treatment Process Variations and Modifications," U.S.
Environmental Protection Agency, Pacific Northwest Lab.,
Corvallis, Oregon.
11.	Burd, R. S. , "A Study of Sludge Handling and Disposal, "
prepared for U. S. Dept. of the Interior, FWPCA, Publication
#WP-20-4, (May 1968).
12.	Loehr. R. C., "Aerobic Digestion - Factors Affecting Design, "
Paper Presented at 9th Great Plains Sewage Works Design
Conference, (March 196 5).
13.	Barnhart, E. L., "Application of Aerobic Digestion to Industrial
Waste Treatment," Proc. of the 17th Purdue Industrial W^ste
Conference, 126-135, (1962).
14.	Smith, A.R., "Aerobic Digestion Gains Favor," Water and
Wastes Engineering, Vol. 8, No. 2, 24-25, (1971).
15.	Benedict, A. H., Carlsen, D. A., "Temperature Acclimatization
in Aerobic Bio-oxidation Systems" Journal-Water Pollution
Control Fed., Vol. 45, No. 1, 10, (January 1973).
16.	"Process Design Manual for Upgrading Existing Wastewater
Treatment Plants, " prepared for U. S. Environmental Protection
Agency by Roy F. Weston, Inc., Contract No. 14-12-933,
(October 1971).
A-3

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APPENDIX B
ADDITIONAL REFERENCES
1.	Dreier, D. E., "Aerobic Digestion of Solids, " Proc. of the
18th Purdue Industrial Wastes Conference, 123-140, (1963).
2.	Jaworski, N., Sawton, G. W., and Rohlich, G. A., "Aerobic
Sludge Digestion," Internation?l Journal Air and W^ter Poll.,
Vol. 4, 106, (1961).
3.	Eckenfelder, W. W. Jr., "Studies on the Oxidation Kinetics of
Biological Sludges, " Sewage and Industrial Wastes, Vol 28,
983, (August 1956).
4.	Reynolds, T. D., "Aerobic Digestion of Waste Activated Sludge,"
Water and Sewage Works, Vol. 113, 37-43, (1967).
5.	Jenkins, D. , Garrison, W E. , "Control of Activated Sludge by
Mean Cell Residence Time, " Journal-Water Poll. Control Fed.,
Vol. 40, No. 11, 1905, (November 1968).
6.	Lawrence, A.W., McCarty, P. L., "Unified Basis for Biological
Treatment Design and Operation," Journal of the Sanitary Engi-
neering Division, ASCE, No. SA 3 Proc. Paper 7365, 757-777,
(June 1970).
7.	Walker, J. D., "Aerobic Digestion of Waste Activated Sludge,"
Presented at the Ohio Water Pollution Control Conference,
Cleveland, Ohio (June 15. 1967).
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